(written from a Production point of view)
According to "It's Federation Day!", a newspaper article created as background material for Star Trek Generations, Natha Kell was the Tellarite ambassador at the Federation Charter signing ceremony on October 11, 2161. The clipping stated that, prior to the conference, he was involved in some type of dispute with Ambassador Sarahd of Andoria and that Kell was famous for declaring, "We defy anyone, even the Romulans, to test our resolve now for collective security."
Although the article containing this information was present within the Picard family album during filming on Generations, neither the character of Natha Kell nor the rest of the article is considered canon, because the article was never seen on-screen.